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Class 8th-Science: Chapter 2-Microorganisms: Friend and Foe Ncert

Q1.  Fill in the blanks.

        (a)  Microorganisms can be seen with the help of a _________.

        (b)  Blue green algae fix _________ directly from air and enhance fertilit of  soil.

        (c)  Alcohol is produced with the help of ________.

        (d)  Cholera is caused by _________.

Ans.  (a) microscope, (b) Nitrogen gas, (c) yeast, (d) Bacteria

Q2.  Tick the correct answer.

        (a)  Yeast is used tn the production of

               (i)  sugar

               (ii) alcohol

               (iii) hydrochloric acid

               (iv) oxygen

        (b)  The following is an antibiotic

               (i) Sodium bicarbonate

               (ii) Streptomycin

               (iii) Alcohol

               (iv) Yeast

        (c)  Carrier of malaria-causing protozoan is

                (i) female Anopheles mosquito

               (ii) cockroach

               (iii) housefly

               (iv) butterfly

        (d)  The most common carrier of communicable diseases is

               (i) ant

               (ii) housefly

               (iii) dragongly

               (iv) spider

        (e)  The bread or idli dough rises because of 

               (i) heat

               (ii) grinding

               (iii) growth of yeast cells

               (iv) kneading

        (f)   The process of conversion of sugar into alcohol is called

               (i) nitrogen fixation

               (ii) moulding

               (iii) fermentation

               (iv) infection

Ans. (a) (ii), (b) (i), (c) (i), (d) (ii), (e) (iii), (f) (iii)

Q3.  Match the organisms in Column A with their action in Column B.

                        A                          B
(i) Bacteria(a) Fixing nitrogen
(ii) Rhizobium(b) Setting of curd
(iii) Lactobacillus(c) Baking of bread
(iv) Yeast(d) Causing malaria
(v) A protozoan(e) Causing cholera
(vi) A virus(f) Causing AIDS
(g) Producing antibodies

Ans.  (i) (e), (ii) (a), (iii) (b), (iv) (c), (v) (d), (vi) (f)

 Q4.  Can microorganisms be seen with the naked eye? If not, how can they be  seen?

Ans.  No, we can not see microorganisms with naked eye. They can be seen with the help microscope.

Q5.  What are the major groups of microorganisms?

Ans.  The major groups of microorganisms are bacteria, fungi, protozoa and some algae.

Q6.  Name the microorganisms which can fix atmospheric nitrogen in the soil.

Ans.  Rhizobium, some bacteria and blue green algae can fix atmospheric nitrogen in the soil.

Q7.  Write 10 lines on the usefulness of microorganisms in our lives.

Ans. The usefulness of microorganisms is as follows:

  1. Lactobacillus, a bacterium is useful in the formation of curd.
  2. Yeast is used in the baking industry for making breads, pastries and cakes.
  3. Yeast is also used for the commerical production of alcohol and wine.
  4. Microorganisms like bacteria and fungi are used to make antibiotics. Examples of these antibiotics are streptomycin, tetracycline and erythromycin.
  5. Antibiotics are mixed with the feed of livestock and poultry to check microbial infection in animals.
  6. Antibiotics are also used to cantrol many plant diseases.
  7. Microorganisms are also used to make vaccines.
  8. They are used to degrade the harmful and smelly substances and thereby clean up the environment.
  9. They are used to make manures.
  10. Some bacteria and blue green algae can fix nitrogen from atmosphere to enrich soil with nitrogen and increase fertility.

 Q8.  Write a short paragraph on the harmful effects of microorganisms.

Ans.  Some of the microorganisms causes diseases in human beings, plants and  animals. Some microorganisms also spoil food, clothing and leather. Many communicable diseases such as cholera, common cold, chicken pox,                    tuberculosis etc. are caused by microorganisms. Protozoans cause serious diseases like dysentery and malaria. Female Anopheles mosquito carries the parasite of malaria. Female Aedes mosquito acts as carrier of dengue virus.        Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus. Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and        other. The diseases reduce the yield of crops. Food poisoning is also caused by microorganisms. They make food poisonous by producing toxic substances in it.      

Q9.  What are ntibiotics? What precautions must be taken while taking antibiotics?

Ans. Antibiotics:

        The medicines that kill or stop the growth of the disease-causing microorganisms are called antibiotics.

        Examples of some antibiotics that are made from bacteria and fungi are streptomycin, tetracycline and erythromycin. 

        The precautions that can be taken while taking antibiotics are as follows:

  1.  These medicines should be taken only on the advice of a qualified doctor.
  2.  One must complete the course prescribed by the doctor.
  3.  If antibiotics are taken when not needed or in wrong does, it may make the drug less effective when it might be needed in future.
  4. Also antibiotics taken unnecessarily may kill the beneficial bacteria in the body.
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